This Firebase Storage error occurs when the local file being uploaded is modified, deleted, or saved again while the upload is in progress. Firebase uses the Blob.slice() API to split files into 256KB chunks for resumable uploads, and detects when the underlying file has changed mid-upload.
The "storage/cannot-slice-blob" error in Firebase Storage indicates that the client-side file being uploaded was altered during the upload process. Firebase Storage uses the standard Blob.slice() API to divide files larger than 256KB into chunks for resumable uploads, which provides better reliability and progress tracking. When Firebase attempts to slice the file during chunked upload, it detects that the file's content or properties have changed since the upload started. This could mean the file was deleted, overwritten, saved again by another process, or moved to a different location. When Firebase detects this inconsistency, it aborts the upload to prevent uploading incomplete or corrupted data. This is a protective mechanism that prevents silent failures where users wouldn't know that their file wasn't properly uploaded due to concurrent file modifications.
The most reliable fix is to create a stable copy of the file that won't be modified during upload:
// Web - Create Blob copy
import { ref, uploadBytes } from 'firebase/storage';
async function uploadFile(storage, filePath, file) {
// Create immutable copy using slice()
// This creates a new Blob with the same content
const stableBlob = file.slice(0, file.size, file.type);
const storageRef = ref(storage, filePath);
const result = await uploadBytes(storageRef, stableBlob);
return result;
}
// Usage
const fileInput = document.getElementById('file-input');
fileInput.addEventListener('change', async (e) => {
const file = e.target.files[0];
try {
await uploadFile(storage, `uploads/${file.name}`, file);
console.log('Upload successful');
} catch (error) {
console.error('Upload failed:', error);
}
});For React with file input:
import { useState } from 'react';
import { ref, uploadBytes } from 'firebase/storage';
export function FileUploader({ storage }) {
const [uploading, setUploading] = useState(false);
const handleFileSelect = async (e) => {
const file = e.target.files[0];
if (!file) return;
setUploading(true);
try {
// Create immutable copy
const stableBlob = file.slice(0, file.size, file.type);
const storageRef = ref(storage, `uploads/${file.name}`);
await uploadBytes(storageRef, stableBlob);
console.log('Upload successful');
} catch (error) {
console.error('Upload failed:', error.message);
} finally {
setUploading(false);
}
};
return (
<input
type="file"
onChange={handleFileSelect}
disabled={uploading}
/>
);
}Creating a copy ensures the original file can be modified by other processes without affecting the upload.
Switch to resumable uploads which handle interruptions better and include retry logic:
import { ref, uploadBytesResumable } from 'firebase/storage';
function uploadWithResume(storage, filePath, file, maxRetries = 3) {
// Create stable copy first
const stableBlob = file.slice(0, file.size, file.type);
const storageRef = ref(storage, filePath);
const uploadTask = uploadBytesResumable(storageRef, stableBlob);
return new Promise((resolve, reject) => {
uploadTask.on(
'state_changed',
// Progress
(snapshot) => {
const progress = (snapshot.bytesTransferred / snapshot.totalBytes) * 100;
console.log(`Upload is ${progress.toFixed(0)}% done`);
},
// Error
(error) => {
if (error.code === 'storage/cannot-slice-blob') {
console.error('File was modified during upload');
}
reject(error);
},
// Complete
() => {
console.log('Upload completed');
resolve(uploadTask.snapshot);
}
);
});
}
// Usage in React
async function handleUpload(e) {
const file = e.target.files[0];
try {
await uploadWithResume(storage, `uploads/${file.name}`, file);
} catch (error) {
console.error('Upload failed:', error);
}
}Resumable uploads allow you to pause and resume if needed, and handle the slicing internally.
Don't upload files that are in temporary locations or being actively managed by the system:
// WRONG - Uploading from temporary directory
const tempFile = '/tmp/upload-cache/image.jpg';
// System may clean this up during upload
// CORRECT - Copy to stable location first
import { ref, uploadBytes } from 'firebase/storage';
import { promises as fs } from 'fs'; // Node.js
async function uploadStableFile(storage, sourceFile, storagePath) {
// For web: use Blob from input
// For Node.js: read file into buffer
const fileBuffer = await fs.readFile(sourceFile);
const storageRef = ref(storage, storagePath);
await uploadBytes(storageRef, fileBuffer);
}
// For mobile (React Native/Flutter):
// Use file:// URLs that point to app-managed directories
// NOT: file system temporary directoriesUse stable file locations:
- Web: Files from <input type="file"> are stable during session
- Node.js: Read entire file into memory before uploading
- Mobile: Use app-managed directories, not system temp folders
If creating files before upload, wait for write operations to fully complete:
import { ref, uploadBytes } from 'firebase/storage';
// Wait for file to be fully written
async function uploadAfterWrite(storage, fileData, fileName) {
// Simulate file creation (image generation, export, etc.)
const createdBlob = new Blob([fileData], { type: 'application/json' });
// Add delay to ensure file system is done
await new Promise(resolve => setTimeout(resolve, 100));
// Create stable copy
const stableBlob = createdBlob.slice(0, createdBlob.size, createdBlob.type);
// Now safe to upload
const storageRef = ref(storage, `uploads/${fileName}`);
const result = await uploadBytes(storageRef, stableBlob);
return result;
}
// For Node.js file operations
const fs = require('fs').promises;
async function createAndUpload(storage, filePath, storagePath) {
// Write file
await fs.writeFile(filePath, 'file content');
// Wait a moment
await new Promise(resolve => setTimeout(resolve, 100));
// Read into buffer for upload
const buffer = await fs.readFile(filePath);
const storageRef = ref(storage, storagePath);
await uploadBytes(storageRef, buffer);
}Ensure all file write operations are complete before attempting upload.
Mobile platforms may lock files during processing. Handle this properly:
// React Native with file picker
import DocumentPicker from 'react-native-document-picker';
import { ref, uploadBytes } from 'firebase/storage';
import RNFS from 'react-native-fs';
async function uploadFromGallery(storage) {
try {
const result = await DocumentPicker.pick({
type: [DocumentPicker.types.images],
});
// Copy file to app's documents directory
const appDir = RNFS.DocumentDirectoryPath;
const fileName = result.name;
const newPath = `${appDir}/${fileName}`;
// Copy from picker result to stable location
await RNFS.copyFile(result.uri, newPath);
// Wait for copy to complete
await new Promise(resolve => setTimeout(resolve, 200));
// Read into buffer
const fileData = await RNFS.readFile(newPath, 'base64');
const blob = new Blob(
[Buffer.from(fileData, 'base64')],
{ type: result.type }
);
// Create stable copy
const stableBlob = blob.slice(0, blob.size, blob.type);
// Upload
const storageRef = ref(storage, `uploads/${fileName}`);
await uploadBytes(storageRef, stableBlob);
// Cleanup
await RNFS.unlink(newPath);
} catch (error) {
console.error('Upload failed:', error);
}
}For Flutter:
import 'package:firebase_storage/firebase_storage.dart';
import 'package:image_picker/image_picker.dart';
import 'dart:io';
Future<void> uploadFromGallery() async {
final ImagePicker picker = ImagePicker();
final XFile? image = await picker.pickImage(source: ImageSource.gallery);
if (image != null) {
try {
File imageFile = File(image.path);
// Verify file exists and is accessible
final exists = await imageFile.exists();
if (!exists) {
throw Exception('File not found');
}
// Upload directly - Firebase SDK handles slicing
await FirebaseStorage.instance
.ref('uploads/${image.name}')
.putFile(imageFile);
} catch (e) {
print('Upload failed: $e');
}
}
}Mobile platforms require careful file handling to avoid system locks and temporary directory cleanup.
Ensure you're using the latest Firebase SDK with all improvements and fixes:
# Web
npm update firebase
# React Native
npm update @react-native-firebase/storage
# Flutter
flutter pub upgrade firebase_storage
# Node.js
npm update firebase-admin
# Verify versions
npm list firebase
npm list @react-native-firebase/storage
flutter pub outdatedCheck your current versions:
// Web
import { SDK_VERSION } from 'firebase/app';
console.log('Firebase SDK:', SDK_VERSION);
// Admin
const admin = require('firebase-admin');
console.log('Admin SDK:', admin.SDK_VERSION);Recent versions include:
- Improved Blob.slice() error handling
- Better file change detection
- More reliable chunk upload logic
- Android: Fixed file locking issues (v11.3.1+)
- iOS: Improved file access patterns (v10.18.0+)
### How Firebase Chunks Large Files
Firebase Storage automatically chunks files over 256KB using the Blob.slice() API:
- Files <= 256KB: Single upload request
- Files > 256KB: Split into 256KB chunks, uploaded sequentially or in parallel
- Each chunk is individually checksummed and verified
When Firebase calls blob.slice(start, end), if the underlying file has changed, the slice operation may fail or return inconsistent data, triggering the "cannot-slice-blob" error.
### Platform-Specific Behavior
Web Browser:
- File objects from <input type="file"> are generally stable
- Blob slicing works reliably on standard browsers
- Issue occurs when uploading programmatically created Blobs from mutable data
Node.js:
- No native Blob support - Firebase uses Buffer wrapper
- File stream operations must complete before upload
- Cannot slice files being actively written
Android:
- File content URIs may point to temporary locations
- System may revoke file access permissions during upload
- Fix: Copy file to app's cache directory first
iOS:
- Document picker files may be locked by the system
- File cleanup on app backgrounding can cause issues
- Fix: Copy to app Documents directory before upload
React Native:
- Different behavior on iOS vs Android
- File URIs from image picker are temporary
- Use native file system copy before upload
### Debugging File Change Issues
Enable detailed logging:
// Monitor file state during upload
async function debugUpload(storage, file) {
console.log('Initial file state:', {
name: file.name,
size: file.size,
type: file.type,
lastModified: file.lastModified
});
const stableBlob = file.slice(0, file.size, file.type);
const storageRef = ref(storage, `uploads/${file.name}`);
const uploadTask = uploadBytesResumable(storageRef, stableBlob);
uploadTask.on(
'state_changed',
(snapshot) => {
// Log every state change
console.log('Upload state:', snapshot.state);
console.log('Progress:', snapshot.bytesTransferred, '/', snapshot.totalBytes);
// Check original file still accessible
if (file.size !== stableBlob.size) {
console.warn('WARNING: File size changed during upload!');
}
},
(error) => {
console.error('Upload error:', error);
if (error.code === 'storage/cannot-slice-blob') {
console.error('The file was modified or became inaccessible');
}
}
);
}### When cannot-slice-blob is NOT the Root Cause
Sometimes the error message appears but the actual issue is:
- server-file-wrong-size: File size mismatch (different fix)
- invalid-checksum: Data corruption during transmission (retry)
- Network timeout: Connection interrupted (retry with backoff)
All should be handled with retry logic, but "cannot-slice-blob" specifically requires file stability.
### Workarounds for Persistent Issues
If file instability is unavoidable:
1. Upload via Cloud Function: Have client send to Cloud Function, function handles Storage upload (server-side is more stable)
// Client sends to function
const uploadFunction = functions.httpsCallable('uploadFile');
await uploadFunction({ data: fileBase64, name: fileName });
// Cloud Function receives and uploads to Storage
exports.uploadFile = functions.https.onCall(async (data, context) => {
const buffer = Buffer.from(data.data, 'base64');
await admin.storage().bucket().file(`uploads/${data.name}`).save(buffer);
});2. Compress before upload: Reduces file size and upload time, less chance of modification
// Compress with pako or browser native APIs
import pako from 'pako';
const compressed = pako.gzip(originalData);
const blob = new Blob([compressed], { type: 'application/gzip' });
await uploadBytes(storageRef, blob);3. Read entire file to memory first: Guarantees immutability during upload (not viable for huge files)
const buffer = await file.arrayBuffer();
const blob = new Blob([new Uint8Array(buffer)], { type: file.type });
await uploadBytes(storageRef, blob);messaging/UNSPECIFIED_ERROR: No additional information available
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